Exemplary embodiments of the present relate to a chopper disc for a device for the processing of neutron beams, the chopper disc being made of carbon fibers. Such a chopper disc has a concentric, hollow cylindrical recess for receiving a hub for connection to a pivot bearing. To absorb neutrons striking the chopper disc, a concentric absorber area is provided. At least one window is provided in the absorber area through which neutrons of the neutron beam can pass.
So-called choppers are used for the processing of neutron beams, which—similarly to X-rays—are used for the translumination and analysis of material specimens. Rotating choppers or chopper discs break a continuous neutron beam down into temporally and spatially limited pulses. The number of pulses and the pulse duration can be established by the number and structure of the windows provided in the absorber area. Neutron pulses of a defined energy form (“color”) can be filtered out by cascading synchronously rotating chopper discs.
It is desirable for the operation of the device for the processing of neutron beams to be able to operate the chopper disc at speeds that are as high as possible. Contactless magnetic bearings are therefore used as pivot bearings that enable speeds of greater than 20,000 revolutions per minute without lubricant under continuous operation and maintenance-free in the radiation field.
For stability-related reasons, chopper discs of carbon fibers are used beginning at a speed of about 15,000 revolutions per minute. As the speed increases, the need arises to increase the strength of the chopper discs more and more. However, the integration of a greater number of carbon fibers for the purpose of increased strength increases the weight of the chopper disc, which runs contrary to greater rotary speed.
Chopper discs of carbon fibers are manufactured from prepared, trimmed fiber mats. The carbon fibers run parallel to each other in each fiber mat. The carbon fibers are pre-saturated or subsequently impregnated with a plastic resin. To manufacture a chopper disc, several of the fiber mats, generally at least four layers, are placed one over the other, the carbon fibers of each layer being rotated with respect to the preceding layer by a prescribed angle. In general, several packages of four layers each are used to enable the forces occurring at the abovementioned high speeds to be absorbed.
One drawback of chopper discs manufactured in this way is that, due to the plurality of carbon fibers, they are relatively heavy, which limits the maximum speed. What is more, due to the rotational forces (centrifugal force) occurring during rotation, it is always only a portion of the carbon fibers that contributes to the strength.
Exemplary embodiments of the present invention are directed to a structurally and/or functionally improved chopper disc allowing use at higher rotational speeds. Exemplary embodiments of the invention are also directed to a device and a method for the manufacture of such a chopper disc.
According to a first aspect of the present invention, a chopper disc for a device for the processing of neutron beams involves chopper disc made of carbon fibers. The chopper disc has a concentric, hollow cylindrical recess for receiving a hub for connection to a pivot bearing. A concentric absorber area is provided in order to absorb neutrons striking the chopper disc. For example, the absorber area can be provided in the form of a ring in a section of the chopper disc that is arranged radially on the outside. At least one window is provided in the absorber area through which neutrons of the neutron beam can pass. The number and structure of the windows is selected according to the needs of the device for the processing of neutron beams. According to the invention, the carbon fibers extend from the outer periphery of the chopper disc nearly radially in the direction of the recess and contact the recess tangentially.
The carbon fibers thus run substantially radially in relation to an axis of rotation of the chopper disc. This optimized fiber layer results in both the greater strength and reduced weight of the chopper disc. Another advantage is that, as a result of the proposed type of arrangement of the carbon fibers, chopper discs of nearly the same strength can be made available in which the ratio of disc thickness to occurring stresses is optimized. As a result, such a chopper disc can be operated at greater rotational speeds.
More precisely, each of the carbon fibers of the chopper disc is composed of two straight sections and a circular arc section. The straight sections contact the recess tangentially with the end that faces toward the recess. The circular arc running between the two straight sections section corresponds in its profile substantially to the edge or the periphery of the recess of the chopper disc.
Expediently, at least a large portion of the carbon fibers extends from the outer periphery of the chopper disc radially in the direction of the recess and contacts the recess tangentially. In another specific embodiment, all of the carbon fibers extend from the outer periphery of the chopper disc nearly radially in the direction of the recess and contact the recess tangentially. This results in the desired optimized fiber layer, which provides greater strength with reduced weight.
In one embodiment, each first and each second end of the carbon fibers ends at the outer periphery of the chopper disc. A section running in the middle between the respective first and second end describes a circular are whose profile corresponds substantially to the edge or periphery of the recess. Each of the carbon fibers therefore has three sections: a first straight section, a second straight section and an arched section, which connects the first and the second sections to one another. For example, the (external) angle enclosed between the first straight section and the second straight section is between 180° and 270°. A high level of stability can be ensured in this way. At the same time, it is ensured that an accumulation of material in the area of the recess, i.e., of the hub arranged there later and of the mutually crossing carbon fibers remains in the inner area. As a result, the areas lying radially on the outside remain protected from excessive forces at the desired high speeds. The ideal angle between the straight sections and hence the profile (or circumferential angle) of the arched section depends on the number of carbon fibers integrated into an inventive chopper disc and, particularly, of superposed carbon fiber layers. The ideal angle can be determined in an optimization process or through experimentation.
In relation to a tangent contacting the periphery of the chopper disc, each first and second end (or the first and second straight section) contacts the tangent associated with it approximately tangentially. The crossing of the first and second end (or the first and second straight section) and the associated tangent is merely approximately orthogonal because, due to the presence of the recess for receiving the hub, the carbon fibers cannot run through the center, i.e., the axis of rotation, of the chopper disc. In order to come as close as possible to the ideal radial profile, the straight sections of the carbon fibers contact the periphery of the recess tangentially.
Each of the carbon fibers can have no, one, two or more points of intersection with other carbon fibers. This depends on whether the straight sections of an associated carbon fiber each have the same or a different (external) angle in relation to another carbon fiber.
In a cross-section on a plane on which an axis of rotation of the chopper disc lies, the thickness of the chopper disc decreases continuously starting from the axis of rotation in the direction of the outer periphery. Because the chopper disc does not have any steps, a high level of strength can be achieved compared to chopper discs made of fiber mats, and with reduced weight. It is particularly expedient in this context if the chopper disc has a convex arch on each side in cross-section at least in part. The cross-sectional shape or the profile of the cross-section of the chopper disc is created not only by the process of laying the individual carbon fibers, but also particularly by pressing with a pressing tool. The shape of the chopper disc and the profile of the outer surfaces follow the shapes of the pressing tool. Consequently, the pressing tool has concave indentations in order to achieve the convex arch.
The carbon fibers are expediently embedded into a resin system or enclosed by the resin system. An example of a resin system that can be used is HEXION L20 with a hardener, e.g., EPH 960. As will become clear from the method described below for the manufacture of the chopper disc, the carbon fibers can already be saturated in resin before “laying” or be infiltrated with resin after laying.
In one specific embodiment, the diameter of the chopper disc is between 50 cm and 80 cm. The diameter of the hub can be from 50 mm to 100 mm. The fiber volume content can be between 50% and 70%.
The at least one window can be provided in a section bordering the outer periphery of the chopper disc. The window or windows can be introduced into the chopper disc using a milling or cutting process after manufacture thereof.
According to a second aspect of the present invention, a winding mandrel is proposed for the manufacture of a chopper disc as described above. The winding mandrel comprises a cylinder whose axial length is very much smaller than the radius. The radius cylinder is somewhat larger than the diameter required for the chopper disc to be manufactured. For example, the axial length can be smaller by a factor of ten. The winding mandrel further comprises two dome top caps arranged on the opposing end of the cylinder, each having an inwardly oriented concave indentation. The cylinder and the two dome top caps can be integrally formed. As a result of the respectively inwardly oriented concave indentation of the two dome top caps, the axial length of the cylindrical winding mandrel is shorter in the area of an axis of rotation than in the area of the outer periphery. Finally, the winding mandrel comprises a concentric recess passing through the cylinder and the dome top caps in the axial direction of the winding mandrel through which a shaft for receiving a particularly divided shaft passes for connection to a drive.
The winding mandrel is a component of a device for the manufacture of the chopper disc and makes it possible, unlike when prepared, trimmed fiber mats are used, to wind an endless carbon fiber onto the winding mandrel in order to simultaneously obtain two of the chopper discs as described above. In particular, the use of a winding mandrel and the winding method associated therewith makes it possible to produce chopper discs whose thickness decreases continuously and thus without steps starting from the axis of rotation in the direction of the outer periphery. As a result, chopper discs having nearly the same strength and with an optimum ratio of disc thickness to stress can be wound.
In one embodiment of the winding mandrel, a needle crown or a slip-resistant surface is provided on the outer periphery of the cylinder. This makes it possible to place a carbon fiber laid on a first of the dome top caps over the cylindrical winding mandrel on the other of the two dome top caps. This ensures that the carbon fiber cannot slip in the radial direction. The slip-resistant surface can also be provided, for example, by means of an appropriate material, e.g., a runner or a porous surface.
As mentioned, the winding mandrel has a diameter that is somewhat larger than the diameter of the chopper disc to be achieved. According to one particular embodiment of the winding mandrel, the diameter is between 55 cm and 90 cm. The axial length is between 5 cm and 20 cm, for example.
According to a third aspect of the invention, a device is provided for the manufacture of a chopper disc of the type described above. The device comprises a winding mandrel as described above. The device further comprises a shaft that is guided through the recess of the winding mandrel and is connected in a positive and/or nonpositive manner to the winding mandrel. A drive is provided in order to enable the shaft to be caused to rotate. Furthermore, the device comprises a carbon fiber feeding device that can be moved relative to the winding mandrel and is embodied so as to deliver an endless carbon fiber. Finally, the device comprises a control device that controls the speed and angle of rotation of the winding mandrel relative to the movement of the carbon fiber feeding device in a predefined manner in order to wind the endless carbon fiber around the winding mandrel.
Through the interplay of the named components, it is possible to wind an endless carbon fiber around the winding mandrel. Two of the above-described chopper discs with their advantageous characteristics are thus produced in a first preliminary stage.
In order to enable the winding to be reliably performed in the manner described above, it is expedient if the shaft has a cone shape that expands away from the shaft on both sides of the winding mandrel. This ensures that the large number of carbon fibers guided around the recess of the winding mandrel or around the shaft are pressed in the direction of the winding mandrel and cannot slip loosely into an undefined position.
According to another embodiment, a unit for saturating the endless carbon fiber with a resin system is provided to wet or soak the carbon fiber with the resin system before the winding process. Alternatively, the wetting or soaking of the carbon fibers can also be done after the winding of the winding mandrel.
According to another embodiment, the device comprises two pressing discs, each of which has a concave arch corresponding to the concave arch of the winding mandrel and a diameter corresponding to the winding mandrel in order to be pushed after the winding process from the opposing sides of the winding mandrel onto the shaft and braced with each other. The pressing discs are used to perform a pressing of the endless carbon fiber wound around the winding mandrel in order to eliminate possible gaps occurring during winding and to ensure a symmetrical shape of the chopper disc. This is ensured by the corresponding sheath shapes of the dome top caps of the winding mandrel and of the two pressing discs.
The pressing is preferably done in an autoclave, in which the curing of the arrangement braced with the pressing discs also occurs in a vacuum atmosphere.
According to a fourth aspect of the present invention, a method is provided for the manufacture of a chopper disc as described above using the device described above. The method comprises the following steps:
a) Winding the carbon fiber delivered endlessly from the carbon fiber feeding device according to a predetermined deposition plan around the winding mandrel, the carbon fiber being guided during each winding step tangentially on the shaft and over the needle crown or the slip-resistant surface in order to produce a chopper disc on both sides of the winding mandrel:
b) pressing the carbon fiber by pushing a respective pressing disc onto the shaft on the opposing sides of the winding mandrel;
c) severing the carbon fiber in the area of the periphery of the winding mandrel in order to obtain two chopper discs, each of which was produced between the winding mandrel and the associated pressing disc, the severing being performed during pressing;
d) curing the still-pressed carbon fiber in an autoclave in a vacuum according to a predefined time, pressure and temperature profile;
e) removing the pressing discs;
f) producing the outer contour and of the at least one window in each of the chopper discs.
According to another embodiment of the method, the manufacture of the edge and of the at least one window is performed by water jet cutting or milling.
The carbon fibers can be saturated with the resin system by or in the carbon fiber feeding device. Alternatively or in addition, the resin system can be introduced into the carbon fiber roving after the winding process. It is preferably introduced before the pressing process.